It is now commonly accepted that the vision impairing disease known as cataracts can be alleviated by surgically replacing the natural lens of the eye with an artificial intraocular lens.
Intraocular lenses have two principle parts: a medial, light focusing body (also called the optic), made of a non-toxic plastic material which will replace the natural lens of the eye and focus light on the retina, and haptic support portions which extend from the optic to the anatomy of the eye and provide means for fixing and holding the optic in proper position within the eye.
The optics for intraocular lenses are made principally by two processes. Some optics are made by a lathe cutting process whereby cylindrical pieces of plastic are placed on a lathe in an appropriate fixture and the optical surfaces of the optic are cut by conventional lathe cutting techniques. After the cutting is accomplished the edges of the optic may be polished by a variety of polishing practices, including tumbling in an abrasive medium. Although lathe cutting procedures provide satisfactory optical lenses, they are time consuming, labor intensive and therefore, expensive to make. Other intraocular lenses are formed by molding. The molding is accomplished by using a rectangular mold insert, like that shown in FIG. 1, with a cylindrical bore 6 extending through insert 1 from one opposite surface to the other. Molding pins 2 and 3, are inserted into opposite sides of mold insert 1 and into bore 6 and spaced apart at a required distance to provide a molding cavity 4. Molten plastic is directed into molding cavity 4 through a gate in the side of mold insert 1.
Molding pins are made of very hard steel and their confronting surfaces are highly polished to provide good optical quality to the molded optic. In order to polish them properly it is necessary to place the pins in a skirt, like that shown in FIG. 2, and then polish right across the interface between the skirt and the pin. It has previously been found very difficult to place any fine molding detail close to the edge of the pin because of the tendency of the pin to fracture at the edge of the pin.
As a result, the sharp edges of the molded optic have to be removed by subsequent machining and polishing of the edges. Great care must be taken to protect the delicate optical zone of the optic during this final edge finishing.
It would be desirable to mold the optic with finished molded radii at the edges which require only minimum subsequent finishing.
Haptic supports can be formed with a thin filamentary resilient unitary memory retaining plastic fibers, for example polypropylene or some other suitable plastic fibrous material, which is resilient and has good spring-like quality so that it may be easily deformed and return to its original shape when the deforming forces are reduced. It has been necessary to make the optic thick enough so holes could be drilled radially into the edge to receive haptic supports. Haptics may be attached to the optic by bonding, gluing, crimping or otherwise fastening the haptic filament into the hole, by a variety of well known processes.
It would be desirable to make the central optical zone of the optic thinner so that the optical properties could be improved and weight of the optic could be reduced but still leave enough material at the edge of the lens to support the haptics.
It would also be desirable to space the posterior surface of the optic away from the adjacent anatomy of the eye to provide room to perform a capsulotomy should it be necessary after the lens is inserted in the capsular bag of the eye. The possible need for a capsulotomy is well-known in this art, and its details will not be discussed further in this application. In the past, certain lenses have accomplished this spacing by adding an annular ridge to the posterior surface of the lens. In certain prior art lenses, this ridge has been molded on the back of the optic by machining a groove 7 around the periphery of the posterior pin 3 as shown in FIG. 1a. However, adding a ridge to the back of the optic makes the optic even thicker and still leaves a relatively sharp edge between the edge on the optic which requires further finishing.
It would be desirable to be able to recess part of the posterior surface of the optic at the edge of the optic to support the haptic and at the same time to provide finished molded radii on the edges of the optic which require only minimum subsequent finishing.